People observe ancient artifacts at the Iraqi National Museum in Baghdad on March 15, 2015, after its reopening in the wake of the recent destruction of archaeological sites by Islamic State. (Karim Kadim/AP)

BAGHDAD — Islamic State militants have provoked a global outcry by attacking ancient monuments with jackhammers and bulldozers. But they also have been quietly selling off smaller antiquities from Iraq and Syria, earning millions of dollars in an increasingly organized pillaging of national treasures, according to officials and experts.

The Islamic State has defended its destruction of cultural artifacts by saying they are idolatrous and represent pre-Islamic cultures. Behind the scenes, though, the group’s looting has become so systematic that the Islamic State has incorporated the practice into the structure of its self-declared caliphate, granting ­licenses for digging at historic sites through a department of “precious resources.”

The growing trade reflects how Islamic State fighters have entrenched themselves since seizing the Iraqi city of Mosul a year ago Wednesday, in a dramatic expansion of the territory they control in this country and neighboring Syria.

The extremist group’s recent capture of Syria’s majestic 2,000-year-old ruins at Palmyra threw a spotlight on the risk that the Islamic State poses to the region’s rich cultural heritage. It is, however, just one of 4,500 sites under the group’s control, according to the Paris-based Financial Action Task Force.

“They steal everything that they can sell, and what they can’t sell, they destroy,” said Qais Hussein Rasheed, Iraq’s deputy minister for antiquities and heritage.

A calcareous stone statue is displayed at the Iraqi National Museum in Baghdad. (Hadi Mizban/AP)

“We have noticed that the smuggling of antiquities has greatly increased since last June,” he added, referring to the month in which Islamic State militants took control of Mosul and large parts of northern Iraq.

At that time, militants also seized the ancient Assyrian capital of Nineveh. In a video released earlier this year, the Islamic State showed its fighters drilling off the faces of the mighty stone-winged bulls on the gates of the city. The militants also filmed themselves destroying statues at Mosul’s museum. But many of those items were actually replicas of antiquities kept in Baghdad, Iraqi officials said. Anything genuine and small enough to move was likely sold off or stockpiled by the militants, they said.

Iraq has suffered from years of despoilment of its historic sites, as thieves have taken advantage of instability in the country. The sacking of the poorly guarded National Museum in Baghdad after the U.S.-led invasion in 2003 was decried around the world.

The Islamic State’s plundering began in a haphazard fashion when the extremists first gained a foothold in Syria. But the trade has become more organized as the group has conquered territory.

The Islamic State grants licenses for the excavation of ancient sites through its “Diwan al-Rikaz” — a governing body for overseeing resources in the “caliphate.” The body has a department for oil and gas, as well as antiquities, documentation from the group shows.

“Islamic State has incorporated the activity of excavation into its bureaucracy,” said Aymenn al-Tamimi, a researcher on jihadist groups at the Britain-based Middle East Forum who has compiled an archive of Islamic State administrative documents.

Video from April 2015 purports to show Islamic State militants destroying the ancient city of Nimrud in Iraq. (Reuters)

How much the Islamic State earns from the trade is difficult to estimate. Iraqi officials say it is the group’s second most important commercial activity after oil sales, earning the militants tens of millions of dollars.

With the extremist group struggling to maintain its oil revenue in the wake of U.S. airstrikes that damaged infrastructure, experts and officials worry that the Islamic State — also known as ISIS or ISIL — might focus even more on illegal excavations.

“It’s a dependable source of revenue, which makes it very attractive, and it’s surprisingly untapped,” said Michael Danti, a professor of archaeology at Boston University. “Over time, we’ve seen ISIL and organizations like it increase their ability to draw revenue from these crimes.”

Danti, who advises the State Department on the trade in plundered antiquities from Iraq and Syria, said some looted items have made their way to U.S. and other Western markets — especially antiquities in the lower- and medium-price ranges, such as stone seals with cuneiform inscriptions.

Larger, more conspicuous items will probably go through a laundering process that takes years and involves the forging of documents to suggest a legal provenance. To be traded legally, the items must have been excavated or exported before 1970, when a UNESCO convention came into force prohibiting trade in such cultural property. But the market is poorly regulated and the Archaeological Institute of America estimates that as many as 90 percent of classical artifacts in collections may be stolen antiquities.

Smaller items from Iraq and Syria are now “flooding the ­market” and are widely sold online, said Deborah Lehr, the co-founder of the Antiquities Coalition, which aims to end “cultural racketeering.” She said her organization has been e-mailed by brokers selling smuggled antiquities who mistakenly took her group for collectors.

“There needs to be better education and better regulation,” she said. “The public needs to know that by purchasing these items, people are potentially funding terrorism.”

From Iraq, antiquities are smuggled to Kuwait, Israel and Turkey, all regional transit hubs, said Col. Firas Hussein Abed, an Iraqi army commander who oversaw an investigation into a smuggling ring in April.

A raid by his officers led to the arrest of four people and the recovery of 25 items, which experts are examining to ascertain whether they came from Islamic State-controlled sites.

Around Mosul, there are reports of widespread digging for ancient objects to sell.

Amr al-Julaimi, a lecturer in Mosul University’s antiquities department until it was closed by the Islamic State, said residents have informed him that the group is excavating areas around the tomb of Jonah, the prophet famed in Islam and Christianity for being swallowed by a whale. The tomb was destroyed last July by the group, which deemed it idolatrous.

“The longer until Mosul is liberated, the more the danger that our human legacy will be wiped out,” he said.

One indicator of how rapidly the illegal trade may be growing is the number of declared imports into the United States of antiquities said to be excavated long enough ago to be legal to trade.

The value of antiques and ancient artifacts from Iraq imported into the United States jumped fourfold between 2010 and 2014, reaching more than $3.5 million, according to U.S. International Trade Commission figures. Imports from Syria and Egypt have also skyrocketed.

“It’s highly suspicious,” said Danti, the Boston University professor. “These spikes in supposed legal imports perfectly correlate with the breakdown in law and order in these countries. I’d be shocked if all of it was legal.”

In Iraq, Rasheed advocates a blanket ban on the trade in antiquities from the country. In the same compound as his office is the National Museum, recently reopened to the public for the first time since 2003 in what officials described as an act of defiance against Islamic State destruction.

“The Iraqi people need to be able to witness their history, their diversity,” Rasheed said. “What’s happening is a tragedy.”